This invention relates to valve apparatus for actuating a flow of an ultra high purity gas from a source thereof (as used herein, the term xe2x80x9cultra high purityxe2x80x9d means a gas having a total impurity of 50 parts per million or less, and the term xe2x80x9cgasxe2x80x9d is intended to embrace gas mixtures as well as nuclear gases).
Ultra high purity gases are usually provided in storage cylinders, for use in various processes in which purity is an essential requirement, in the electronics manufacturing industry, for example. Actuation of a flow of gas from the cylinder to the manufacturing apparatus in which the gas is to be used is effected by a valve, having an inlet connected to the cylinder and an outlet connected to a gas conduit leading to the manufacturing apparatus. The principal function of such a valve is effectively to allow or to prevent the flow of gas, rather than adjustably to control the flow.
Such valves have several rigorous requirements. Firstly, the valves must be leak tight; they must prevent the ingress of ambient air, or any other surrounding atmosphere, which would contaminate the ultra high purity gas. Equally undesirable is any leakage of the ultra high purity gas to atmosphere, since many such gases are toxic and/or pyrophoric. Leakage from such valves is particularly problematic due to wear; actuation of the flow depends at some stage on the contact between sealing faces, which are prone to wear, and hence leak, over time.
The valves must be made of a xe2x80x9ccleanxe2x80x9d material, that is to say that the material in contact with the gas must not release molecules or particles which would act as contaminants. Similarly, the valve must be configured so as to prevent gas flows from entraining molecules or particles of the valve material, through abrasive or erosive action, for example. To help prevent contamination, the flows of gas through the valve should be considered, particularly upon opening the valve; when the valve is first opened, there is a tendency for gas to accelerate towards the valve outlet, and if the flow path of the gas is complicated or tortuous, an accelerating xe2x80x9cplugxe2x80x9d of gas can impact on inner surfaces of the valve, hastening wear and detaching particles of the valve material which contaminate the gas. The valves must also be configured so as to minimise such contamination, and also in order that the purge process is easily and effectively achievable; many manufacturing processes require the use of a sequence of several ultra high purity gases, and it is therefore necessary at separate stages of the process to remove one ultra high purity gas supply and replace it with another, without any contamination of the second gas and without any release of the gas to atmosphere, hence an effective purge is absolutely required.
A corollary of the requirements that the valve not contaminate the gas flowing therethrough and that the valve be easy to purge effectively is that the valve should have a low xe2x80x9cwetted areaxe2x80x9d, that is, the surface area of that part of the valve which comes into intermittent contact with the gas or the flow is actuated should be minimised. A low wetted area is important as it reduces the available surface area of valve from which particles might be carried to contaminate the gas flow, and it reduces the available surface area of valve to which gas molecules might temporarily bond, thereby making the purging process more difficult.
Accordingly, the present invention provides a valve for use with an ultra high purity gas comprising a valve body defining a valve chamber, having a valve outlet through which gas is discharged and containing a reciprocable sealing member, characterised in that the valve chamber is in fluid communication with a source of the gas and in that the sealing member is reciprocable along an axis substantially parallel to the direction of flow of gas discharged from the valve outlet and into and out of sealing contact with a sealing face located within the valve chamber and circumscribing the inlet end of a valve outlet pipe, the opposite, outlet end of which pipe forms the valve outlet.
With such an arrangement the wetted area is only that defined by the valve outlet pipe, between its inlet and outlet ends, which can be minimised. The valve chamber may be in constant contact with the gas, however this is acceptable because this volume does not need to be purged. Because the sealing face contacts a peripheral seal, which is surrounded by the gas in the valve chamber, the flow of gas on opening the valve accelerates into the valve outlet pipe, and provided that this is designed with no unnecessary constrictions or bends the initial xe2x80x9cplugxe2x80x9d of gas will not impact on the valve material, thus avoiding the detaching of particles which could contaminate the gas. Precisely because the valve outlet pipe is configured to permit free flow of gas therethrough, also means that the valve outlet pipe and the area of the reciprocable sealing member circumscribed by the valve outlet pipe sealing face (the xe2x80x9cwettedxe2x80x9d areas) are easily purged. The surface of the valve outlet pipe exposed to gas flowing therethrough can easily be formed with (or of) a suitable material to minimise reaction with and/or contamination of the gas, and the entire arrangement can be constructed so to be at least as leak tight as conventional valves.
Very preferably, the outlet end of the valve outlet pipe is circumscribed by a sealing face for sealing against means for conducting gas discharged from the valve for subsequent use.
In this way, a gas conduit leading to an apparatus in which the gas is to be used can be sealingly attached to the valve with ease. The valve outlet pipe is advantageously an integral member which is separable from the valve, to permit easy replacement. Thus, by exchanging a single part of the valve, all those valve parts subject to wear (namely the sealing faces at the inlet and outlet ends of the valve outlet pipe) can be swiftly and easily replaced.
The valve outlet pipe, or at least the internal part thereof through which gas flows, is preferably generally cylindrical, so that the inlet and outlet sealing faces thereof are easily manufactured. Also, screw thread fittings for fixing the valve outlet pipe into the valve are easily manufactured or fitted on a cylindrical body. The outlet pipe is made of a material and has a surface finish such that any reaction between it and the gas is minimised.
The valve chamber may be substantially annularly disposed about the inlet end of the valve outlet pipe so that, as the valve is opened and the sealing member moved away from the sealing face at the inlet end of the valve outlet pipe, gas has a relatively free flowpath into the valve outlet pipe thereby promoting uniform gas flow thereinto and therethrough and so minimising possibly harmful impact and/or abrasion by the gas flow on the inner surface of the valve outlet pipe.
The sealing member is preferably attached to a resilient diaphragm, which is configured and adapted to seal off the valve chamber opposite the valve outlet. As is known in the art, an actuator is provided behind the diaphragm to reciprocate the sealing member, so as to open or close the valve, whilst flexing the diaphragm but maintaining the valve chamber sealed relative to the ambient atmosphere.
The valve chamber may be at least partially defined by a lining member releasably contained within the valve body and the reciprocable sealing member. This lining member may be easily exchanged as may be necessitated for reasons of wear, or to prevent adverse reaction and to optimise performance should the valve be used with a different kind of gas.
In order to minimise contamination arising from the cylinder in which the gas is supplied, a dip tube may extend from the valve chamber into the cylinder interior, the dip tube being adapted to allow gas to flow freely, via a bell-mouth for example, from the cylinder, from a point distant from the internal cylinder walls. Dip tubes are known in the art.